ShadowDepthWrapper: Fix a memory leak when new effects must be created

[Popov72]

This is the PR #14406 by @nonlinearthink with a fix for the Webgl warnings (see #14406 (comment)).

[bjsplat]

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[bjsplat]

Snapshot stored with reference name:
refs/pull/14449/merge

Test environment:
https://babylonsnapshots.z22.web.core.windows.net/refs/pull/14449/merge/index.html

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[sebavan]

Thanks a lot @nonlinearthink for finding the leak !!!